Focusing plate for magnetic mass spectrometer

ABSTRACT

A magnetic mass spectrometer including an ion source with electrodes to provide an acceleration of an ion beam is described. A magnetic analyzer disperses the beam according to the mass of the individual ions with a collector located downstream for providing a measure of the magnitude of the ion beam for a selected mass. An electric field generating means which may include three sets of deflector plates is positioned adjacent the collector for attenuating or compensating for aberrations of the ion image, for example, the aberrations which results from the non-uniformity of the magnetic field in the region near the magnet boundary. The deflection voltage applied to the deflector plates may be a set ratio of the accelerating voltage and depends upon the size and placement of the plates and the distance of ion beam travel after deflection along with its lateral displacement.

United States Patent [191 Brubaker FOCUSING PLATE FOR MAGNETIC MASS SPECTROMETER [76] Inventor: Wilson M. Brubaker, 1954 Highland Oaks Dr., Arcadia, Calif. 91006 (22] Filed: May 5, I972 [21] Appl. No.: 250,686

Primary ExaminerWalter Stolwein Assistant Examiner-B. C. Anderson Attorney, Agent, or FirmHarold L. Jackson; Stanley R. Jones; Joseph W Price Mar. 26, 1974 [5 7] ABSTRACT A magnetic mass spectrometer including an ion source with electrodes to provide an acceleration of an ion beam is described. A magnetic analyzer disperses the beam according to the mass of the individual ions with a collector located downstream for providing a measure of the magnitude of the ion beam for a selected mass. An electric field generating means which may include three sets of deflector plates is positioned adjacent the collector for attenuating or compensating for aberrations of the ion image, for example, the aberrations which results from the non-uniformity of the magnetic field in the region near the magnet boundary. The deflection voltage applied to the deflector plates may be a set ratio of the accelerating voltage and depends upon the size and placement of the plates and the distance of ion beam travel after deflection along with its lateral displacement.

9 Claims, 6 Drawing Figures FOCUSING PLATE FOR MAGNETIC MASS SPECTROMETER BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates generally to mass spectrometers and more particularly to magnetic mass spectrometers.

2. Description of the Prior Art Basically, magnetic mass spectrometers are ion optical instruments with momentum dispersion. When ions of all masses are accelerated through a common potential difference, the instrument exhibits mass dispersion. For a given ratio of object to image distance, the mass dispersion is proportional to the instrument radius.

Molecules of the sample to be analyzed must be ionized. Ionization is accomplished by any one of several means. The most common means are by electron or photon bombardment. After formation, the ions are accelerated and focused into a beam by fields which result from the application of potential differences between electrodes of the ion source.

The most common means of separating ions for analysis makes use of a uniform magnetic field wherein the ions follow curved paths whose radii are mass dependent. Another function of the magnetic field is to form an optical image of the ion exit slit of the source.

The mass resolved ion beam which passes through the resolving slit impinges upon an electrode which may be either a Faraday cup or the conversion electrode of a secondary electron multiplier. By varying a parameter to which the mass of the transmitted ions is dependent, such as the accelerating voltage or the magnetic field, ions of each mass present are caused to pass through the resolving slit, onto the collector electrode. If the mass determining parameter is varied continuously, ions of differing masses are caused to pass through the resolving slit in a sequential manner. If the ion currents of the various ions are recorded, a mass spectrum is obtained. Two important properties of the magnetic analyzer are its ability to form ion optical images and to provide mass dispersion which causes the images of beams of ions of differing masses to be spatially separated at the image plane.

In common with all optical instruments, the magnetic sector forms imperfect images. The geometric or ideal image is broadened by aberrations. For many geometrical configurations, the Berry curvature aberration is dominant, see Image Curvature Caused by Fringing Fields in Magnetic Sector Mass Spectrometers," by Clifford E. Berry, the Review of Scientific Instruments, Volume u e a a ss. -85 Qs ohst. 1956. The passage of ions through the fringing fields of a magnetic sector causes the image of a line object to be curved. This results because the rays of ions off the median plane traverse a fringing field of a slightly different configuration from that traversed by those on the median plane. Generally the Berry aberration will be evidenced by a curvature of the focused ion beam in the image plane so that in cross section it appears as a curved shape. A

The Berry curvature results from the fact that the trajectories of ions which lie. in the mid plane differ slightly from the trajectories of ions which lie in the planes near the magnetic pole pieces. The trajectory differences are due to the differences in the distribution of the magnetic field intensity in the fringe region,

which lies mainly beyond the geometric boundaries of the magnet pole pieces. The ions which enter the magnet on the mid plane encounter the fringe field before those which approach the magnet on a parallel path, displaced toward one of the pole pieces. This circumstance is reversed as ions on different planes leave the magnet.

Although the theory developed in the above article by Clifford E. Berry makes a first order prediction of the contours of the beam cross section, in practice it is found that if the resolving slit is to be curved to match the beam cross section, the appropriate curve is best obtained empirically. Although not understood, the existence of the curved image has been known since 1942. The work on the Calutrons, magnetic isotope separators, utilized resolving slits that were made to conform to the Berry curvature to improve the resolution power capabilities of the instrument. The curvature of the slit boundaries were determined empirically by copying the image that the ion beam produced on a photographic plate.

A typical width of a mass resolving slit is 2 mils. The fabrication and the assembling of a pair of curved mass resolving slits of this size is most difficult and is subjective to the particular instrument. Further, the correction is of a fixed nature and cannot be varied in relationship to the other parameters of the instrument. In addition to the Berry aberration, there may be other aberrations, such as a lack of alignment of the ion optical image and the resolving slit. This lack of alignment may result from non-uniformities of the magnetic field, or manufacturing errors in the placement of the ion source or resolving slits.

SUMMARY OF THE INVENTION The present invention permits a correction of the aberrations of images such as the Berry aberration, by the use of electric field generating means such as deflector plates which, when appropriately energized, produce electric fields which deflect the ion beam to greatly attenuate the aberrations of the ionic image. The deflection voltages applied to the deflector plates can be set ratios of the ion accelerating potential. The resulting device permits a reduction in the mass of the instrument by as much as one half while maintaining the same sensitivity and resolving power.

The features of the present invention which are believed to be novel are set forth with particularity in the appended claims. The present invention, both as to its organization and manner of operation, together with further objects and advantages thereof, may best be understood by reference to the following description, taken in connection with the accompanying drawings.

BRIEF DESCRIPTION OFTI-IE DRAWINGS FIG. 5 is a schematic drawing of the relationship between the deflection plates and the resolving slit.

DESCRIPTION OF THE PREFERRED EMBODIMENT:

Referring to FIG. 1 and FIG. 3, a variable DC voltage source 2 is provided for energizing both the ion source 12, and the deflector plates 20, 22, and 24. A vacuum source such as an ion pump 8 maintains a pressure in the order of 10 torr so that most ions pass through the spectrometer without colliding with gas molecules. The vacuum source also provides a means for continuously removing the admitted sample.

A magnet 14 is positioned in the ion beam path 7 and is controlled by a magnet control 16. The magnet 14 produces a field which curves the trajectory of the ion beam path 7. A collector in the form of a Faraday cup 15 receives the ion beam current through a resolving slit 25. The current received by the collector 15 is amplified by an amplifier 17 so that it can be recorded on the recorder 19.

The sets of deflector plates 20, 22, and 24 are preferably positioned immediately in front of the collectors resolving slit 25, see FIG. 5, and are biased by voltages that are set ratios of the DC scanning or accelerating potential 2. While numerous methods can be utilized to provide these set ratios of voltages, for purposes of illustration, the potentiometer 30 is disclosed. The average of the potentials applied to each set of deflector plates is zero. This results from the fact that the current drawn by the ion source is negligible relative to the current through the potential divider 28, and the fact that R, and R are equal. Although only one potentiometer 30 is shown for deflector plates 20, it should be clear that a separate potentiometer should be provided for each separate set of deflector plates.

Deflector plates 20 and 24 are biased in one direction while the central deflector plate 22 is biased in the other direction. Referring to FIG. 3, it can be seen that the image 40 exhibits Berry curvature of radius R which can be attenuated by energizing the deflector plates 20, 22, and 24 with the appropriate potentials. The end portions of the image are moved relatively to the right by the fields provided by deflector plates 20 and 24 while the central portion is moved relatively left by the deflector plate 22 to form a line image 42.

The deflections are independent of the mass of the ions when the deflection voltages are proportional to the kinetic energies of the ions. This is accomplished automatically by making the deflection voltage portions of the ion accelerating potential. In traversing an electric field which results from a deflecting potential V placed upon the deflector plates having a length L in the direction of the optic axis separated by a distance D, an ion beam which has been accelerated through a potential difference V is deflected through an angle (1) of 1/2 (L/D) (V /V radians. If, after this deflec tion, the beam travels a distance of X at constant energy, its lateral displacement x is given by (l) Eliminating d),

deI/ ucc (2) This equation (2) provides the ratio of voltage to be applied to the deflecting plates 20, 22, and 24 to move the image a distance x when the mid points of the deflecting plates are a distance X from the resolving slit '25 as in FIG. 5.

Referring to FIGS. 2a and 2b, A denotes the plane of the trajectory of a portion of the ion beam 7 which traverses the median plane of the magnet, while B denotes the plane of the trajectory of a portion of the beam 7 which traverses the magnet in a plane nearer a pole boundary. FIG. 4 discloses the relative intensity of flux along ion path 7 in the vicinity of the magnet edge. 8 represents the flux density in a median plane while B represents the flux density along an ion trajectory adjacent a pole piece. FIG. 2b shows the manner in which ionic paths in each of the planes A and B are deflected differently as a consequence of the differing distributions of the fields, as depicted in FIG. 4. The paths are assumed to be parallel inside the magnet and they are parallel as they leave the magnet. The ion paths are displaced relative to each other owing to the differences in the field distribution illustrated in FIG. 4. This difference in ion paths is the manifestation of the Berry aberration. By application of the deflection voltage V on deflecting plates such as 20, the portion of the ion beam which lies in the general vicinity of deflection plates 20 is given a transverse deflection which causes the curved beam boundaries to become straightened as the beam proceeds to the resolving slit.

The exact placement of the deflector plate sets 20, 22, and 24 along the ion beam path 7 is arbitrary as long as the above deflection voltage current equation is satisfied.

The use of the teachings of this disclosure becomes of increased importance as the quality of the optic image improves. In particular, it is of great importance in connection with instruments of the double focusing type. These instruments are highly corrected for the second order effects of variations in angular distribution, a, and of velocity variationsfi. In tandem instruments of the Nier-Johnson geometry where a direction focus is formed between the sectors, the deflector plates may be placed near the focal plane. However, a preferred place of location is immediately before the resolving slit between the magnetic sector and the focal plane of the instrument. In this case, X becomes small. Since the deviation, x is in the order of 10 inches or less, it is apparent that the plates will function properly if there is a physical space in front of the resolving slit in which they may be placed. Generally, L/D will be in the order of unity, and thus it can be seen that the deflector voltage becomes a very small fraction of the ion accelerating potential.

The effect of the deflection plates 20, 22, and 24 are of a first order consideration and the simple set of electrodes proposed here will not produce an image which is strictly rectangular with straight sides. However, it is estimated that the use of the deflector electrodes 20, 22, and 24 will reduce the Berry aberration contribution to the beam width by a factor of at least percent. As an example of the value of this refinement, it has been noted that in a particular instrument, the Berry aberration broadens the beam by 1 mil when the geometric image is only 1.4 mils wide. Hence, the reduction produced by the present invention amounts to a significant amount. Based on measured beam widths, it appears that this invention would reduce the beam width by about 25 percent based on the above assumptions. If the beam width is reduced by 25 percent, the same sensitivity and resolving powers can be obtained if the instrument dimensions in the x-y plane are reduced by the same factor, leaving the slit sizes the same. Thus, a 25 percent scale size change in two directions results in about a 50 percent reduction in the mass of the instrument.

When the Berry aberration effect is controlled by the deflector plates of the present invention, various combinations of electric and magnetic sectors may be used which offer special advantages of sector angle, size and cost.

The improved performance obtained through the use of the image straightening techniques of the present invention can be used in a number of different ways. First, the instrument can be made appreciably smaller and provide the same performance because a small instrument radius is required to provide the spatial separation of the straightened ion images. When the image heighth is maintained, the instrument volume and mass decreases as the square of the radial dimension changes thus affecting a substantial cost saving. On the other hand, with the elimination of the image defects, the same resolving power is achieved by using larger slits at higher sensitivity. Higher resolving power may be obtained through the use of a narrower resolving slit with a sharpened image thereby providing the high resolving power with the same sensitivity. Or, ajudicious combination of these benefits may be chosen.

The appropriate deflection potentials can be obtainedby a feedback arrangement such as a computer circuit 13 shown in FIG. 1 which automatically adjusts the deflection potentials by means of an electric motor 44 which drives the potentiometer wiper arms 46 through a suitable mechanical linkage, not shown. The computer circuit 13 drives the motor 44 and wiper arms 46 until the width of the ion beam is minimized at the resolving slit. The potentiometers of each set of deflector plates can be controlled in a similar manner as shown in FIG. 1. When this is done, the time varying image degradion by surface charges may be attenuated.

Further, if the mass spectrum is to be scanned magnetically by the magnet control 16, the normalized distribution of the magnetic field becomes a function of the field intensity, owing to saturation effects in the edges of the pole pieces. This variation changes the Berry aberration. The computer circuit 13 follows these changes and inputs the appropriate correction signals to re-position the ganged potentiometer 30. Conversely, the computer may make adjustments to the deflection voltages in a manner which responds to the intensity of the magnetic field. The field intensity may be measured directly by the magnetic transducer 31 or it may be approximated by a signal from the magnet controller 16.

The deflection plates of the present invention are also useful to correct other image defects. In particular, there may be a misalignment of the source and object slits which would increase the apparent beam width because the image would cross the resolving slit at a skew angle. This defect can be corrected by applying asymmetric potentials to the upper and lower sets of deflectors which results in a rotation of the ion beam about the optic axis, bringing it into alignment with the resolving slit. Finally, the deflector plates of the present invention provide an aberration attenuation means that is adjustable and not subjective to the particular instrument components.

It should be noted that various modifications can be made to the apparatus while still remaining within the purview of the following claims.

What is claimed is:

l. A magnetic mass spectrometer comprising:

an ion source;

means for producing an ion beam including an accelerating voltage source;

a magnetic analyzer means for dispersion of the ion beam according to momentum;

means defining a resolving slit;

a collector electrode positioned behind the resolving slit for receiving the ion beam; means connected to the collector electrode for generating a signal corresponding to the ion beam;

means for generating a non-uniform electric field traverse to the ion beam for attenuating the aberrations in the ion beam image to minimize the beam width at the resolving slit including at least three sets of deflector plates positioned adjacent the collector in two parallel planes with a respective plate of each pair in a separate parallel plane, and

a voltage source for each set of deflector plates that is a ratio of the accelerating voltage source as follows:

where V deflection voltage applied to deflector plates V,, accelerating voltage D distance between plates L length of plates in direction of ion beam X distance from the mid-point of the deflector plates to the resolving slit and x lateral displacement required of each portion of the ion beam to minimize the width of the total ion beam at the resolving slit.

2. A magnetic mass spectrometer as in claim 1 wherein the electric field generating means, which includes at least three sets of deflector plates positioned adjacent the collector, has a central set of plates having an opposed bias relative to the other two sets of plates.

3. A magnetic mass spectrometer as in claim I further including computer means connected to the means for generating a signal for varying the non-uniform electric field to compensate for image defects produced by surface and space charges and to minimize the ion beam width.

4. A magnetic mass spectrometer as in claim 1 wherein the means for generating a non-uniform electric field includes a computer circuit to automatically minimize the beam width at the resolving slit.

5. A magnetic mass spectrometer as in claim 1 where the magnetic analyzer means includes a variable magnetic control for changing the flux density; means for sensing the field intensity; and a circuit means connected to the sensing means and the means for generating a non-uniform electric field for adjusting the electric field in response to the flux density.

6. A magnetic mass spectrometer as in claim 1 where the magnetic analyzer means includes a variable magnetic control for changing the flux density and a circuit means connected to the variable magnetic control and the means for generating a non-uniform electric field for adjusting the electric field in response to the flux density.

7. A magnetic mass spectrometer as in claim 1 where the magnetic analyzer means includes a variable magnetic control for changing the flux density and a circuit means for varying the non-uniform electric field to compensate for any changes in the aberrations resulting from the change in flux density.

8. A magnetic mass spectrometer comprising:

an ion source;

means for producing an ion beam including an ion accelerating voltage source;

a magnetic analyzer means for dispersing of the ion beam according to momentum including a variable magnetic control for changing the flux density to magnetically scan the ion beam;

means for sensing the magnetic field intensity;

means defining a resolving slit;

a collector electrode positioned behind the resolving slit for receiving the ion beam;

an ion current amplifier connected to the collector electrode for generating a signal corresponding to the ion beam;

means for generating a non-uniform electric field traverse to the ion beam for attenuating any aberrations in the ion beam image to minimize the beam where V deflection voltage applied to deflector plates V accelerating voltage D distance between plates L length of plates in direction of ion beam X distance from the mid-point of the deflector plates to the resolving slit and x lateral displacement required of each portion of the ion beam to minimize the width of the total ion beam at the resolving slit.

9. A magnetic mass spectrometer as in claim 8 wherein the sets of deflector plates are positioned in two parallel planes with a respective plate of each pair in a separate plane. 

1. A magnetic mass spectrometer comprising: an ion source; means for producing an ion beam including an accelerating voltage source; a magnetic analyzer means for dispersion of the ion beam according to momentum; means defining a resolving slit; a collector electrode positioned behind the resolving slit for receiving the ion beam; means connected to the collector electrode for generating a signal corresponding to the ion beam; means for generating a non-uniform electric field traverse to the ion beam for attenuating the aberrations in the ion beam image to minimize the beam width at the resolving slit including at least three sets of deflector plates positioned adjacent the collector in two parallel planes with a respective plate of each pair in a separate parallel plane, and a voltage source for each set of deflector plates that is a ratio of the accelerating voltage source as follows: Vdef/Vacc 2(D/L) (x/X) where Vdef deflection voltage applied to deflector plates Vacc accelerating voltage D distance between plates L lenGth of plates in direction of ion beam X distance from the mid-point of the deflector plates to the resolving slit and x lateral displacement required of each portion of the ion beam to minimize the width of the total ion beam at the resolving slit.
 2. A magnetic mass spectrometer as in claim 1 wherein the electric field generating means, which includes at least three sets of deflector plates positioned adjacent the collector, has a central set of plates having an opposed bias relative to the other two sets of plates.
 3. A magnetic mass spectrometer as in claim 1 further including computer means connected to the means for generating a signal for varying the non-uniform electric field to compensate for image defects produced by surface and space charges and to minimize the ion beam width.
 4. A magnetic mass spectrometer as in claim 1 wherein the means for generating a non-uniform electric field includes a computer circuit to automatically minimize the beam width at the resolving slit.
 5. A magnetic mass spectrometer as in claim 1 where the magnetic analyzer means includes a variable magnetic control for changing the flux density; means for sensing the field intensity; and a circuit means connected to the sensing means and the means for generating a non-uniform electric field for adjusting the electric field in response to the flux density.
 6. A magnetic mass spectrometer as in claim 1 where the magnetic analyzer means includes a variable magnetic control for changing the flux density and a circuit means connected to the variable magnetic control and the means for generating a non-uniform electric field for adjusting the electric field in response to the flux density.
 7. A magnetic mass spectrometer as in claim 1 where the magnetic analyzer means includes a variable magnetic control for changing the flux density and a circuit means for varying the non-uniform electric field to compensate for any changes in the aberrations resulting from the change in flux density.
 8. A magnetic mass spectrometer comprising: an ion source; means for producing an ion beam including an ion accelerating voltage source; a magnetic analyzer means for dispersing of the ion beam according to momentum including a variable magnetic control for changing the flux density to magnetically scan the ion beam; means for sensing the magnetic field intensity; means defining a resolving slit; a collector electrode positioned behind the resolving slit for receiving the ion beam; an ion current amplifier connected to the collector electrode for generating a signal corresponding to the ion beam; means for generating a non-uniform electric field traverse to the ion beam for attenuating any aberrations in the ion beam image to minimize the beam width at the resolving slit including three sets of deflector plates positioned adjacent the collector; circuit means connected to the three sets of deflector plates, the ion current amplifier, and the means for sensing the magnetic field intensity for adjusting the electric field to maximize the signal of the ion beam; and a voltage source for each set of deflector plates that is a ratio of the accelerating voltage source as follows: Vdef/Vacc 2(D/L) (x/X) where Vdef deflection voltage applied to deflector plates Vacc accelerating voltage D distance between plates L length of plates in direction of ion beam X distance from the mid-point of the deflector plates to the resolving slit and x lateral displacement required of each portion of the ion beam to minimize the width of the total ion beam at the resolving slit.
 9. A magnetic mass spectrometer as in claim 8 wherein the sets of deflector plates are positioned in two parallel planes with a respective plate of each pair in a sepaRate plane. 